Drywall to MDF: On studs or offset?

Hello! I have a question I didn't know how to search for and this forum seems friendly and knowledgeable. Background: I am constructing a recording booth on a tight budget that is mostly build for sound control, as I live in the country and angering neighbors is far from an issue. It was a small unfinished basement room that I made into both the live room and control room by building a "room in a room" using standard 2x4 wooden single studs, 1/2" drywall inside, 5/8" drywall outside, fiberglass insulation (cant remember thickness or type, but the price was right). All except for the front wall to the control room (which is the only directly exposed wall to the control room). This wall got extra thick insulation (9" thick crammed between the 2x4s) and 3/4" MDF instead of drywall. Next, I want to green glue and put on another layer of 5/8" drywall directly onto the MDF. My Question is this: Since MDF can handle the load of drywall hanging by screwing directly into it, would it be better to offset the screws from hanging the drywall off from the studs, or simply go directly into the stud instead? Thanx for your input! I am sorry if this question has been asked before, I have googled for a few days and I don't know if there are better words for finding this information.

Hi "no-destiny". Welcome. There is an announcement at the top of the forum about what to do to assure getting as many responses as possible. The announcement leads to this post (click here). Actually, several people, who are experts on this forum, will not reply if you don't do what is written in that post. Many others who are very helpful, will not reply out of respect for the moderators' wishes. Having said that, the following caught my attention:

"room in a room" using standard 2x4 wooden single studs, 1/2" drywall inside, 5/8" drywall outside,

If I understand that right, you built a three-leaf wall? The existing basement wall (one leaf) , plus the leaves on either side of the studs? I'm not sure why you would do that. Since it is an underground basement I guess it doesn't really matter, except that you wasted the drywall on the outside of the studs (facing the original basement walls. It serves no purpose there, and would have been far more useful as a second layer on the inside of the studs, with nothing at all on the outside of the studs.

This wall got extra thick insulation (9" thick crammed between the 2x4s)

Whoa!!!! :!: I hope you mean that you have two separate stud frames with a 9 inch air gap between them, and not that you just crammed 9 inches of fiberglass into the 3 1/2 inch gap between the two drywall faces on the SAME stud frame? Is that what you did? That's what it sounds like! What I mean is: Does that wall consist of only one frame made from 2x4 studs, with a sheet of drywall on each side (or drywall on one side and MDF on the other)? And you stuffed 9 inches of fiberglass into that 3 1/2 gap between the two? If that's what you did, then that wall is probably basically useless, and you wasted a lot of fiberglass. Compressing fiberglass into one third of the volume it was designed for will most likely cause it to become a flanking path between the two leaves of that wall. Of course, if it is a simple stud frame with drywall on each side, then is was already a pretty poor isolator, and the studs themselves are "flanking", but my guess is that cramming three times to much wool in there probably made it even worse, since now the entire panels are flanking all over, instead of just the small area on the studs. Or did I misunderstand your description of the construction?

and 3/4" MDF instead of drywall.

Why MDF instead of drywall? Mass for mass, drywall is usually cheaper than MDF in most places. Two layers of 5/8 drywall would give you more mass for less money, unless MDF is really cheap in your area (or drywall is really expensive).

My Question is this: Since MDF can handle the load of drywall hanging by screwing directly into it, would it be better to offset the screws from hanging the drywall off from the studs, or simply go directly into the stud instead?

It makes no difference. If the MDF is attached directly to the studs, then it is already coupled anyway, so it makes no difference at all where you put the screws. It would only make a difference if you would have installed resilient channel between the studs and the MDF, which would have gained you stacks of isolation. In that were the case, then you would want to be careful to not shirt-circuit the isolation with a stray screw that hit a stud, but in your case the entire wall is already short-circuited and fully coupled, so it makes no difference at all.

Next, I want to green glue and put on another layer of 5/8" drywall directly onto the MDF.

Top be honest, I don't think Green Glue will help much in your situation, if I understand the way you have built things so far. The wall is not decoupled at all, it is packed full of three times too much wool, so I doubt that it will even rate much more than STC-30. If you use GG, and it works magically to the best possible effect seen in their documentation, you still wouldn't get much more than about STC-36 or so for your wall. It hardly seems worthwhile to spend all that money to put GG on a fully coupled wall, for so little benefit. If you would have built a decoupled wall on two sets of studs, then GG would definitely be worthwhile, and you should get something well over STC-60 like that. But from what you wrote, that isn't the case, so GG doesn't seem worthwhile, as far as I can see. - Stuart -

hello Soundman, I read the announcement at the top before I made this post! However, if I missed anything, please feel free to correct me and I will do what I can to fix it. Anywho, to clarify a few things... It basically goes like this on all walls except the front... Cement wall > 3-4" airspace > 5/8" Drywall > Insulation > 1/2" Drywall I'm really new to this and just looked up a few ways how others have done it. The reason I crammed 9" thick fiberglass insulation in the front wall down to the 3 1/2" was since a lot of people seemed to recommend 703 or 705 rigid fiberglass, I couldn't find it locally... so I simply put as much density as possible in there. I have heard of others doing this... I don't see why it wouldn't work since its increasing mass. This is only on the front wall, the side walls got I think 6". Would this change how the "spring" effect works as you first described? Why is multiple "springs" a bad thing? MDF was free ;) I have a friend that works at a mill and takes home most of the rejects. Comes out in my favor! I realize my setup isn't of the greatest for true studio quality sound isolation. Again, this build was in a very small room as it is so I had to make the greatest use of the space, or else I would have done staggered or double stud walls. In a future home, its definitely part of the design. I'm also no expert at this stuff, I just had the chance to make the room into something and since I wanted a vocal booth and maybe throw in some other instruments (just barely too small for anything larger than a child's sized drum kit!). Thanx for your help. Edit: Just a thought. Obviously what is built is built and would be far too much time (and money, really) to go back and redue any of it, but would it be possible to add resilient channel directly to the MDF and do two sheets of 1/2" drywall, greenglued together of course, to the channel? I realize it would be tough not to let screws hit the MDF, but some drilling and careful measuring to prevent that ;)

No Destiny I just read your response, and here is what I believe -

Cement wall > 3-4" airspace > 5/8" Drywall > Insulation > 1/2" Drywall

= M-S-M-S-M = triple leaf = bad You only want M-S-M = Mass- Spring- Mass

The reason I crammed 9" thick fiberglass insulation in the front wall down to the 3 1/2" was since a lot of people seemed to recommend 703 or 705 rigid fiberglass, I couldn't find it locally... so I simply put as much density as possible in there. I have heard of others doing this... I don't see why it wouldn't work since its increasing mass. This is only on the front wall, the side walls got I think 6". Would this change how the "spring" effect works as you first described? Why is multiple "springs" a bad thing?

First of all, when you cram 9" thick fiberglass, into a 3 1/2 wall space, you ARE making it denser, but the insulation needs to act as a spring. Since you crammed this fiberglass, you have taken this spring and turned it into mass most likely. This now defeats your whole purpose of having two masses with a air gap because all the insulation will be doing now is coupling them together. What you really should have shot for, was leaving the air space between the two walls, and insulating the frame with normal R-13, or 19. True some people do use rigid fiberglass, but the gap still needs to be evident in order for the spring to work, and don't forget, insulation is not part of the mass, its part of the spring!

Obviously what is built is built and would be far too much time (and money, really) to go back and redue any of it

That all depends if you can live with the quality of sound that you may or may not get out of this room. My build, which is still on going, took me 3 tries before I even got the framing correct, but sometimes it is very possible to salvage most of the materials, if rebuilding is needed, in your case it would look like a bit more GB.

would it be possible to add resilient channel directly to the MDF and do two sheets of 1/2" drywall, greenglued together of course, to the channel?

This could only benefit you if you opened your wall back up and created the needed air space. RC on your build from which you stated, would only decouple mass, from mass, from mass. I'm no expert, but Im just giving ya a lil heads up, others may chime in here as well - Hope that helps -Ian

Interesting info Native. Thanx! Now, I don't want to sound arguementative, when really I am trying to understand this... why is a "3 leaf" system bad? Does it create some strange resonance or something? Also, if I packed in the figerglass and killed a leaf of it, wouldnt I have made it 2 leaf... and made it better unintentionally? :D Sorry for the questions... Wish I would have asked before the build, but oh well! This is only a temporary setup... hopefully! Edit: Another afterthought... I COULD pull the MDF off pretty easily and put it on resilient channel, then put the 1/2" drywall with GG on it. Would that be recommended? The fiberglass could be held back with something like chickenwire to keep from touching the MDF, yet still keep it compressed in there for the added mass... just a few ideas that I am willing to do at this point. Again, I'm new at this, so sorry if any of this is blasphemy.

I have heard of others doing this... I don't see why it wouldn't work since its increasing mass.

Like Ian said, the purpose of fiberglass is not to add mass, and in any case the small amount that it adds is not enough to be useful. The purpose of insulation on the wall cavity is to damp the high-frequency resonance and flutter-echo between the two leaves of the wall, and also to act as a damper for the "spring" in your Mass-Spring-Mass (MSM) system. By cramming in three times too much, you have probably defeated the purpose of the insulation, since you will have changed the gas flow resistance properties, and it is now three times too dense. The ideal density is about 48 kg/m3, but you have probably pushed that up to around 150 kg/m3. (compared that with 700 kg/m3 for drywall, and 2300 kg/m3 for reinforced concrete, and you get the reason why the mass of your insulation is negligible compared with the mass of the leaves). So basically the insulation is probably not acting as a damper for the spring anymore, (although it might still be doing something for the flutter echo. Hard to say.) The second issue is that by cramming it in so tightly, you have probably also defeated the purpose of decoupling: insulation compressed that much will probably provide somewhat of a flanking path, a "bridge" that directly guides vibrations from one leaf to the other, bypassing the entire air spring.

Would this change how the "spring" effect works as you first described? Why is multiple "springs" a bad thing?

Yes, I suspect that it will. Or more accurately, it will change the function of the "spring-damper" assembly, which is the air plus the insulation. You have changed the gas flow resistance of the "damper", and since the insulation itself occupies volume, then there must be less air in there too, meaning that you have less spring, probably. It's beyond my abilities to figure out how it might actually work, but I can tell you that it won't work the way it is supposed to. Also, you don't have multiple springs due to having more insulation in the cavity: there is still only one spring. The air itself is the spring. You only have multiple springs if you put something solid and massive inside the wall, creating a three-leaf system. In that case you now have two much smaller air gaps, with much thinner springs, which means you have much lower isolation at low frequencies.

MDF was free ;) I have a friend that works at a mill and takes home most of the rejects. Comes out in my favor!

Cool! There is nothing wrong with MDF on your wall: It is mass, after all, and that is what you need. And if it comes for free, then that sure beats the cost of drywall! This part, at least, is not a problem. MDF will work fine as part of an MSM system.

Again, this build was in a very small room as it is so I had to make the greatest use of the space,

Taking off the drywall, taking out two thirds of the insulation, installing RC on the studs, then putting the drywall back again would make a HUGE difference to how your wall isolates. And I mean HUGE. My guess is that you would gain at least 20 dB of isolation like that, perhaps more (RC alone will get you 16 dB, if you do it right). Meaning that you will reduce power transmission through the wall to 1/100 of what it is now (yes, one one-hundredth), implying that it will be at least four times quieter (or one quarter of the sound level) that you are getting now. That's a major gain for just a bit of work. And the RC is not very thick. It won't take up much space.

Obviously what is built is built and would be far too much time (and money, really) to go back and redue any of it,

Not really. It would only take a couple of days to get the drywall off, pull the excess insulation, put up the RC, and put the drywall back again. if you work carefully, you might even be able to re-use a lot of the drywall you take off, provided that you caulk all the old nail holes really well. Your only real cost would be the RC, plus maybe a couple of sheets of drywall. But this time make sure that your insulation exactly fills the gap between the leaves of the wall, just touching the leaves on each side, but NOT crammed in tightly. Or better still, leave a thin air gap on one side, say 1/4" or 1/2", just to be safe. You get maximum isolation when the cavity is filled 100%, but reducing that a little won't do much harm, especially seeing that you already compressed the fiberglass really tight, and it might expand back to it's original size over time.

but would it be possible to add resilient channel directly to the MDF and do two sheets of 1/2" drywall, greenglued together of course, to the channel?

Nope. The air gap between the MDF and the drywall would be far too thin to work. You need about 170 mm of air depth for maximum effect. At 60mm the effect is practically zero. All you would succeed in doing is adding a third leaf, which REDUCE your low frequency isolation, and you would probably also be creating a resonant cavity whose would frequency is way up there in the low mids, right where you don't want it. It might act as a panel absorber of some kind, but it might also just sing along with your songs... Since you do seem to have the budget for another two sheets of drywall, plus RC and GG, then if you really do want good isolation, my suggestion would be to bight the bullet, take off the drywall, pull the excess insulation, put up the RC, put on the drywall, caulk, seal, mud, paint. Done! Afterwards, you'll be glad you did!

Cement wall > 3-4" airspace > 5/8" Drywall > Insulation > 1/2" Drywall

Like Ian said: that's a three leaf. Bad. Take off the inner layers of drywall (so you can see the studs), and you'll basically have what is called an "inside out" wall, which is GREAT for small rooms, since it GAINS you space! You can use the space between the studs for your acoustic treatment, instead of having to hang it on the drywall. It should gain you nearly 8 inches of width and 8 inches of length for your room (4 on each wall)... :) - Stuart -

Now, I don't want to sound arguementative, when really I am trying to understand this... why is a "3 leaf" system bad? Does it create some strange resonance or something?

Take a look at this diagram, to get an idea of different things you can do to a wall, and how they change the isolation: The reason why this happens is rather complex, but basically you have to realize that if you draw a graph of "frequency" versus "isolation", you will find that that the curve is NOT a straight line: It is actually a wobbly curve. There are several sections to that curve, varying by the frequency, each segment is governed by a different mathematical equation, based on a different parameter of your wall, ranging from "stiffness", "resonance", "mass", "coincidence", etc. As you change features in the wall, you change the point on the graph where you switch over from one parameter to the next as being predominant, and governing the isolation. There is a point on that curve where it switches over from being governed by resonance, to being governed by mass law. The point where that happens depends on several factors, but one of the main ones is the depth of the air gap. So if you REDUCE the air gap then you INCREASE the frequency of the point where this switch-over occurs. If you want to think of it in simpler terms: An MSM wall is designed to have a resonant frequency. That's what MSM does: In physics, if you hang a MASS from a SPRING that is in turn attached to another MASS, then you have created a resonant system. Start the lower mass moving, and it will automatically bob up and down (vibrate) at its natural resonant frequency (governed by things such as the stiffness and length of the spring, the mass, etc.). But it will NOT want to vibrate at OTHER frequencies. If you try to force it to vibrate at a frequency OTHER than the natural resonant frequency, it will resist that. It will fight you, as it tries to do what it does best: vibrate at its natural frequency. In short, it acts like a notch filter, or more accurately, a bandpass filter. That is the basic design principle of an MSM wall: it is tuned to a specific resonant frequency, that is as low as possible, outside of the range of human hearing. At that specific frequency, it actually TRANSMITS sound extremely well: it basically BOOSTS sound getting through the wall at that frequency, HELPING it along! It amplifies sound transmission at the resonant frequency. From that frequency on upwards, it gives less and less boost, until at about 1.4 times the resonant frequency it is merely transparent to sound. At about twice the resonant frequency, it starts isolating well, and beyond that, the higher you go in frequency the better it gets. So obviously, you MUST keep the resonant frequency of the MSM system in your wall down to well below the lowest frequency that you want to block. If you add a third leaf, then that "re-tunes" the system to a much higher frequency. So now the wall is "transparent" and even "boosting" at frequencies that are inside the human hearing range. If you add a forth leaf, then you re-tune the system resonance once again, to an even higher frequency, making the problem even worse. That is what you are seeing on the diagram above. That's why the STC rating goes UP from #3 to #4, and up again from #4 to #5. Even though you are taking mass AWAY from the wall in both cases, the STC rating gets BETTER because you are forcing the resonant frequency down lower and lower. In the last case (going from #5 to #6), the STC improves once again, but this time mostly due to mass law: you doubled the mass of the wall, by putting two more sheets of drywall on it. (Mass law says that each time you double the mass, you get a 6 dB boost in isolation). I don't know if that answers your question, but it should give an idea for doing more research. It's hard to get your head around these concepts, as they are NOT intuitive! Maybe this graph will help you get it a bit better:

Also, if I packed in the figerglass and killed a leaf of it, wouldnt I have made it 2 leaf... and made it better unintentionally?

You didn't necessarily kill it completely: You just changed the parameters in unpredictable ways. You "killed" it in the sense that it does not work in the way that it is designed to work, but it might well be doing something totally different and unpredictable, and 99 times out of 100 that means "BAD"!

Another afterthought... I COULD pull the MDF off pretty easily and put it on resilient channel, then put the 1/2" drywall with GG on it. Would that be recommended?

Yup! that would work.

The fiberglass could be held back with something like chickenwire to keep from touching the MDF, yet still keep it compressed in there for the added mass

Nope! That will NOT work. The "added mass" is negligible at best, does nothing for you anyway, and puts you in uncharted territory: You'll have to come up your own equations to calculate what that is happening inside your wall, and you'll have to do your own acoustic laboratory testing, since what you are doing is off the charts that I'm aware of: you are on your own if you go down that path! The modified gas flow characteristics of the insulation are probably doing things to your spring that alter the way it works, alter the resonant characteristics of your wall, and alter the shape of the TL curve: It can probably all be calculated, sure, but you are off the paying field as far as general "rules of thumb" go. Just pull the excess out, get it down to 100% cavity fill at NORMAL density, and you will be fine. With RC, MDF, insulation and drywall, you should get a pretty good wall. But only if you do it the way that has been proven to work, tested in labs all over the world, and for which there are equations, research papers, general rules, and numerous successful studio builds, all over the world.

Again, I'm new at this, so sorry if any of this is blasphemy.

Not quite, but pretty close!! :) - Stuart -

Excellent information! Good old math and science to the rescue! :D I think I will do that then. Whenever I get around to building a house with a good sized basement, I will take note on this thread and revisit it for sure! Makes a lot more sense with diagrams and in-depth, yet simple, explanations. Thanx again!